This invention relates to cathode-ray tubes, and particularly to color cathode-ray tubes of the type useful in home television receivers and color display tubes, and to inline electron guns therefore having a high degree of insensitivity to deflection defocusing of the electron beams.
An inline electron gun is one designed to generate at least two, and preferably three, electron beams in a common plane and to direct the beams along convergent paths to a small spot on the screen. In one type of inline electron gun, such as that shown in U.S. Pat. No. 3,772,554, issued to R. H. Hughes on Nov. 13, 1973, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having the bottoms of the members facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In such electron guns, static convergence of the outer beams with respect to the center beam is usually attained by offsetting the outer apertures in the second focusing electrode with respect to the outer apertures in the first focusing electrode.
An inline electron gun wherein two electrostatic focusing lenses are utilized to form an effective larger main focus lens is described in a copending U.S. patent application Ser. No. 485,860, filed on Apr. 18, 1983, entitled, "Color Picture Tube Having An Improved Inline Electron Gun", by D. J. Bechis, et al., and assigned to the same assignee as the present invention. In the Bechis et al. copending patent application, the third and fifth electrodes from the cathode are electrically interconnected and the fourth and sixth electrodes are electrically interconnected. Facing portions of the fifth and sixth electrodes each include a peripheral rim and three separate inline apertures therein set back from the rim. The peripheral rims are elongated in the inline direction of the inline apertures and form an astigmatic focus field. This field may be matched to an astigmatic beam forming region formed by the first and second electrodes from the cathode.
It has been noted that the horizontal beam landing locations of the outer electron beams, in color picture tubes having the above-described electron guns, change with changes in the focus voltage applied to the electron guns. It therefore is desirable to improve such inline electron guns to eliminate, or at least reduce, this horizontal convergence sensitivity to focus voltage changes.
Copending U.S. patent application Ser. No. 461,584, filed on Jan. 27, 1983 by H. Y. Chen and assigned to the assignee of the present invention, discloses a screen grid structure shown in FIG. 3 of the copending application for reducing the horizontal convergence sensitivity of the inline electron gun to focus voltage changes. The screen grid structure disclosed in the copending Chen patent application utilizes a pair of reconvergence slots formed in the first accelerating and focusing electrode side of the screen grid electrode. The reconvergence slots are formed closely to and inwardly from the outer apertures in the screen grid electrode and cause a refraction of the electrostatic beam path between the screen grid electrode and the first accelerating and focusing electrode to compensate for the offset refraction within the main lens of the electron gun.
An alternative screen grid structure for reducing the sensitivity of the inline electron gun to focus voltage changes is also disclosed in copending U.S. patent application Ser. No. 492,044 entitled, "Cathode Ray Tube Having Asymmetric Slots Formed In A Screen Grid Electrode Of An Inline Electron Gun", filed on May 6, 1983, by H. Y. Chen and assigned to the assignee of the present invention. In the alternative screen grid structure, asymmetric depressions are formed about the outer apertures in the first accelerating and focusing electrode side of the screen grid electrode. In one embodiment, the depressions are transverse slots which also reduce vertical flare which appears on the screen of the tube as an undesirable low intensity tail or smear extending from a desirable intense core of the electron beam. Flare is common in tubes having a deflection angle in excess of 90 degrees.
While the screen grid structures described in the Chen copending patent applications are satisfactory for reducing the horizontal sensitivity of the outer beams with respect to focus voltage changes, a simpler structure that can be easily and inexpensively produced is desired.